Power electronics include a technique which deals having a with power electronic devices to convert and a control electric power and energy, power electronic devices used in on-off mode, and a power conversion system as its application technique.
Conversion of electric power calls for a variety of power semiconductors with switching capability. These semiconductors in practical use include rectifier diodes (with a pn Junction for current flow in only one direction), and thyristors, bipolar transistors, and MOSFETs (with a combination of pn junctions). Recently developed ones include insulated gate bipolar transistors (IGBT) and gate turn-off thyristors (GTO) which perform switching in response to gate signals.
These power semiconductors, evolve heat when energized. They tend to evolve more heat with increased capacity and speed. To protect them from deterioration and a reduced life due to the effects of heat they should be provided with a radiator which prevents temperature rise in the temperature device itself and in the vicinity thereof. A common material used for radiators is copper, which is inexpensive and has a high thermal conductivity (393 W/m). Unfortunately, copper is not suitable for the radiator of power semiconductor devices because it has a high thermal expansivity of 17×10−6/° C. and hence it does not solder well with silicon, whose thermal expansivity is 4.2×10−6/C. One way to address this problem is to make the radiator from molybdenum or tungsten which has thermal expansivity close to that of silicon or to interpose one of these materials between the radiator and the semiconductor element.
Power semiconductor elements are contrasted with electronic semiconductor elements. The latter are exemplified by integrated circuits (IC) consisting of electronic circuits integrally formed on a single semiconductor chip. They are classified into memory, logic, microprocessors, etc. according to their functions. A problem involved with recent electronic semiconductor elements is heat evolution, which increases as the degree of integration increases and the speed of operation increases. To make things worse, electronic semiconductor elements are contained individually in hermetic packages for isolation from the atmosphere to prevent failure and deterioration. Various types of packages including ceramic packages (in which each semiconductor element is fixed to ceramics through die bonding) and plastics packages (which are sealed with plastics) are in widespread use. A new development to meet requirements for high reliability and high speed operation is the multi-chip module (MCM) equipped with a plurality of semiconductor elements on a single substrate.
A plastics package is constructed such that the semiconductor element therein has its terminals connected to the lead frame through bonding wires and the entire assembly is sealed with plastics. Recent improvements made to cope with increasing heat evolution involve packages in which the lead frame functions to dissipate heat or which are provided with a radiator for heat dissipation. The lead frame or radiator for heat dissipation is usually made of copper having a high thermal conductivity. Unfortunately, malfunction can be anticipated because of the difference the thermal expansivity between copper and silicon.
By contrast, ceramics packages are constructed such that a semiconductor element is placed on a ceramic substrate having wiring printed thereon and the entire assembly is sealed with a metal or ceramics cap. The ceramic substrate is backed with a Cu—Mo or Cu—W composite material or a kovar alloy, which functions as a radiator. Ceramic materials with low thermal expansivity, high thermal conductivity, and good workability are required at a low production cost.
An MCM consists of a metal or ceramic substrate having thin film wiring formed thereon, a plurality of semiconductor elements (in the form of a bare chip) mounted thereon, a ceramic package containing the elements, and a sealing lid. The package is provided with a radiator or fin if it needs heat dissipation. The metal substrate is made of copper or aluminum. This device has the advantage of a high thermal conductivity, but also the disadvantage of a high thermal expansivity, which leads to poor matching with the semiconductor element. Therefore, the substrate of MCMs for high reliability is made of silicon or aluminum nitride (AIN). The radiator, which is bonded to the ceramic package, should be made of a material which has high thermal conductivity and also has low thermal expansivity for good matching with the package material.
As mentioned above, all semiconductor devices evolve heat during operation and are subject to malfunction if heat is accumulated. Therefore, they need a radiator with good thermal conductivity for heat dissipation. The radiator, which is bonded to the semiconductor element directly or indirectly through an insulating layer, calls for not only a high thermal conductivity, but also a low thermal expansivity for good matching with the semiconductor element.
Prevailing semiconductor elements are based on the use of Si or GaAS, which have a coefficient of thermal expansion of 2.6×10−6 to 3.6×10−6/° C. and 5.7×10−6 to 6.9×10−6/° C., respectively. Among known materials comparable to them in thermal expansivity are AlN, SiC, Mo, W, and Cu—W. When used alone for radiators, they do not permit their heat transfer coefficient and thermal conductivity to be controlled as desired. They are poor in workability and high in production cost. A Cu—Mo sintered alloy is proposed in Japanese Patent Laid-open No. Hei 8-78578. A Cu—W—Ni sintered alloy is proposed in Japanese Patent Laid-open No. Hei 9-181220. A Cu—SiC sintered alloy is proposed in Japanese Patent Laid-open No. Hei 9-209058. An Al—SiC composite material is proposed in Japanese Patent Laid-open No. Hei 9-15773. These conventional composite materials permit their heat transfer coefficient and thermal conductivity to be controlled over a broad range if the ratio of their constituents is changed. However, they are poor in plastic workability and hence they present when used in making thin plate and need many manufacturing steps.
It is an object of the present invention to provide a composite material having a low thermal expansivity, a high thermal conductivity, and good plastic workability, a semiconductor device made with said composite material, a radiator for said semiconductor device, an electrostatic attractor, and a dielectric plate for said electrostatic attractor.